If organic material can be converted to underground coal then we could store the carbon in our organic waste.
How coal is made in nature
If the organic matter is buried deeply enough, then we have genuine sequestration. A third of it turns to coal in a process known as coalification.
First anaerobic (without air) microorganisms convert it to peat, then as it gets deeper and hotter, chemistry takes over. Any chemical process carried out under pressure will produce a compound that occupies less volume.
One third of a woody plant is lignin made up of benzene rings. These are squeezed together so they join. The cellulose and hemicellulose are reduced to methane.
The methane adheres to micro cracks called cleats, in the coal and is held there by water pressure. If the water is removed then the methane is released. Hence coal seam gas. CO2 can preferentially displace the CH4.
Coalification in the local council tip
In a normal council tip the coalification process starts to happen almost immediately. Biological processes start that will later be continued by heat and pressure as it is buried deeper.
As the aromatic rings are joined together, the aliphaatic chains between them are expelled as mainly methane - CH4.
This drill rig is putting in methane collection wells at Kimbriki tip. The organic material being dug up is already black, and in the first stages of coalification.
The methane is being captured to be burnt for energy. Better to produce CO2 than allowing methane to escape when it has 20 x the warming of CO2.
To produce a total solution, the CO2 would need to be buried as well.
There is a process for making synthetic coal from green biomass. It was developed for producing fuel rather than burying carbon.
It combines biomass with steam at 200oC at 20 bars for a few hours.
Details of the process: SyntheticCoal
Lignin is made of benzene rings with space between them.
Then it is squeezed....
Once squeezed, the benzene rings join together to form coal.
Each bonding releases more methane.
With more pressure the benzene rings continue to join together.